Geology-The Central Wales Orefield

• Northern Ceredigion, North West Powys and South West Gwynedd form a district where there are many disused metal mines, this area is referred to as the Central Wales Orefield.
• The veins were formed during earth-movements and the minerals were deposited from hot fluids that flowed through fractures in the rocks

• At least twelve episodes of fracturing and mineralisation have been identified
• The mineralisation is thought to have taken place episodically from the Devonian Period (beginning 408 million years ago) through to the Permo-Triassic (250 million years ago)
• The veins are hosted by Lower Palaeozoic marine sedimentary rocks, about 430 million years old
• The veins primarily contain ores of lead, zinc, silver and copper mixed with quartz (silica) and calcium, magnesium and iron-bearing carbonate minerals
• The veins also contain small amounts of cobalt, nickel, antimony, arsenic, cadmium and traces of gold.
• All metals, except gold, occur in the form of sulphides (minerals containing metals combined with sulphur)
• The main ores sought by the Miners were Galena, Calchopyrite and Sphalerite

• Galena (left) is an ore bearing Lead & Silver. It is heavy to hold and when freshly broken has a metallic silver grey appearance which glitters in the light. Galena is lead sulphide, with the chemical formula PbS. Although pure lead sulphide contains 86.6% lead the galena found at these mines typically contains numerous small inclusions of other sulphide minerals and an important "impurity", silver. The mines of Central Wales were an important source of silver, and at the richest mines the galena could yield in excess of 30 ounces of silver to the ton of ore.

• Sphalerite (right) was known to the miners as zinc blende or "Black Jack". In its natural state it is most commonly a dark brown colour. The more iron it contains the darker its colour. Sphalerite or zinc-blende, is zinc sulphide with the chemical formula ZnS for the pure form, which contains 66.94% zinc. However, some iron (typically 2-5%) is commonly present, and sphalerite also contains small amounts of cadmium. Although abundant in Central Wales, it was only mined in earnest from the 19th Century onwards, when demand rose for zinc after a practical way to smelt the ore had become well-established. Prior to this time it was tipped with the waste, and at some mines it was later hand-picked from the tips once it became economically viable to do so.

• Chalcopyrite (left) is a copper iron sulphide with the chemical formula CuFeS2. It contains 34.63% copper and is the most important ore of that metal worldwide. Its golden yellow colour may mislead inexperienced prospectors to believe they have found gold! Chalcopyrite is widespread throughout Central Wales, but was only present in workable amounts at a relatively small number of mines, for example in the Cwmerfin district and in the area north of Nant-y-Moch Reservoir in Ceredigion and at Dylife in NW Powys.

• The miners worked mineral veins occurring along geological faults. When mined, the mineralised rock was crushed and the heavy ore minerals separated out using water to wash away the lighter waste material
• The ore minerals were collected and bagged as concentrates which were then sold to smelters
• Mineral production was only recorded in detail from 1845 onwards and even then records are unreliable - sometimes being used to encourage investors generosity!
• It is estimated that more than 450,000 tons of lead ore concentrates were produced containing over 2,500,000 ounces of silver
• It is estimated that more than 140,000 tons of zinc ore and over 8,000 tons of copper ore concentrates were produced
• Local production of barite (barium sulphate) and iron pyrites also took place
• Today, the mines are of major scientific importance for their geology, mineralogy and rare, metal-tolerant plants that grow on the spoil-heaps
• There are other orefields elsewhere in Wales that were known for their production of copper and gold.